The present state of the art is that in many modern electronic devices, parts are bonded together by solder interconnections. The bonded parts are often composed of materials with significantly different coefficients of thermal expansion (CTE). For instance, a ceramic electronic device, with low CTE, may be soldered onto a polymeric printed circuit board with a high CTE.
The problems with the present state of the art are, for instance, problems that often arise due to significant mismatches in CTE. Bending of an electronic device during handling or service can further aggravate these mismatches in physical dimensions between parts.
Typically, solder paste is comprised of solder powder and a flux vehicle. The solder powder is typically an alloy of two or more elements, which melt at a soldering temperature. When mixed with a flux vehicle, a paste is formed. The paste may be printed onto a circuit board using a stencil. Electronic components may be placed on top of the paste and the entire assembly may be placed inside an oven set at a temperature higher than the melting point of the alloy. The solder melts and wets both the component and pads on the board. When the temperature is decreased, the solder solidifies, joining the components to the board with a solid solder joint.
A standard solder joint is a solid piece of solder bonding together two parts. If a solder joint is weak, the joint may rupture quickly and typically results in device and system failure. If a solder joint is soft and compliant, such as when an Indium-Tin compound (e.g. 52In48Sn) is used, it may deform to cope with a small mismatch in the CTE's or physical dimensions between parts. However, in the case of a large mismatch in the CTE's or physical dimensions between parts, even a soft solder joint may be unable to cope and joint rupture typically results.
Contrarily, if a solder joint is very strong, a new problem set arises. For instance, U.S. Pat. No. 5,088,007 uses copper fiber or copper filler in a solder joint to provide further reinforcement on the solder joint strength. This composite solder generally makes the solder joint harder and stronger resulting in fairly durable joints for devices with small mismatches in the CTE's or physical dimensions between parts. However, for devices with large mismatches in the CTE's or physical dimensions between parts, the stresses resulting from these mismatches do not disappear due to the use of strong rigid composite solder joints. For instance, if a solder joint itself is too strong to fail, a device may fail at spots weaker than the joint, such as at the bonding between solder and pads, the bonding between solder and devices, or the body of a board/device itself, in order to relieve the stress.
The fatigue resistance of solder joints for a system with mismatches in the CTE's or physical dimensions between parts can be improved by increasing joint standoff. For instance, U.S. Pat. No. 6,360,939 describes a way to increase the standoff between a component and a substrate. In this process, high melting temperature metal particles are mixed with lead-free solder powder (approximately 3–10% of the total metal weight), and then a solder paste is made using this metal mixture. The high melting temperature metal particles give the solder a larger standoff value for the substrate than normal solder powder allows, purportedly resulting in an increase in fatigue life. However, the validity of this approach is compromised by the particle size allowed for in an associated dispensing or printing process. Large high melting temperature metal particles do not allow adequate solder paste dispensing or printing. On the other hand, small high melting temperature metal particles do not provide sufficient standoff, such that the solder joint is unable to cope with a large mismatch in the CTE's or physical dimensions between parts. The end result is a solder joint that is solid. Much like concrete, the solder is strengthened by the filler material, resulting in a more rigid joint. Although this rigid joint offers some advantage for typical electronic products, the rigidity could cause joint or component cracking particularly in the case of a large mismatch in the CTE's or physical dimensions between parts.
U.S. Pat. No. 6,340,113 ('113) describes a process for producing a solder joint without melting down whole solder constituents. The purpose is to eliminate slump and spread of solder paste during the soldering process by avoiding the melting of solder particles. Purportedly, this very porous joint reduces the potential for joint fatigue. The method of producing a solder joint in this patent relies on solid state diffusion, in which a tin particle is coated with a thin layer of lead. When heated at a temperature below the melting temperature (232° C.) of tin, tin atoms diffuse into the outer lead layer and forms a very thin layer of tin-lead alloy with the tin content continuously increasing from 0% up to a high content. This thin layer of tin-lead alloy exhibits a solidus temperature of 183° C. for most of the composition formed by solid-phase diffusion. If the soldering process is maintained at a temperature higher than the solidus temperature, the thin outer tin-lead layer on the tin particle remains at a pasty or liquid state. Upon contact with neighboring tin particles, the thin outer liquid layer could coalesce and form a liquid solder bridge between neighboring tin particles. When the joint is cooled down, tin particle aggregates are bonded together by a thin layer of tin-lead solder to form a network of connected particles. Within this network, inter-particle spaces remain and a joint is formed with some porosity.
The '113 method is very labor intensive and requires the coating of metal particles with a second metal surface by plating, which is both time consuming and cost prohibitive. The '113 patent also requires a maximum temperature below the melting temperature of tin (232° C.) and lead (327° C.), but above the solidus temperature (183° C.) of tin-lead alloy. In the electronic industry, it is common practice to heat solder paste up to a temperature at least 20° C. above the melting temperature of solder, or 203° C. in the case of tin lead solder (e.g., 63Sn37Pb) in order to allow sufficient solder wetting and form an acceptable solder joint, leaving a 29° C. processing window (between 232° C. and 203° C.) for tin-lead solder systems. However, if a solder joint with a prevailing lead-free solder of tin-silver-copper (melting temperature 217° C.) is to be formed, the maximum temperature must be below the melting temperature of tin, but at least 20° C. above that of the tin-silver-copper (217° C.) surface coating on the tin particles. In other words, the maximum temperature (232° C.) allowed to retain the solid state of the tin would be below the minimum temperature (237° C.) needed to get sufficient solder wetting of the tin-silver-copper alloy, effectively leaving no process window for main-stream lead-free tin-silver-copper solder systems. The impracticalities of the '113 approach to lead-free solder systems virtually eliminates its application in the electronic industry.
Due to the toxicity of lead, lead is to be globally banned from use in the electronic industry on Jul. 1, 2006. Therefore, new solders are required to be lead-free if they are intended to be used beyond Jul. 1, 2006.
When solder paste is deposited by printing or dispensing, and then reflowed between a substrate and an electrical component having a large mismatch in the CTE's or physical dimensions between parts, joint cracking or fatigue failure typically results either right after the soldering process or during service. Therefore, there is a need for compositions and methods to relieve joint strain through a porous solder joint which exhibits a high elasticity and low rigidity while maintaining formable characteristics through a regular soldering process.